Alot of this is mind games to be quite frank. I said alot, not all, as there really is some fantastic juice out there being sold by vendors. With that in mind, let me explain.
When you're browsing a website and it's all beautifully decorated with big shiny buttons to click and nice wide attractive text sitting below a vivid, saturated picture that makes your mouth water poetically describing how the flavors that combine to make this recipe start off with 'A ripe, sun-dried white peach' and finish off with 'An ever so slight touch of refreshing sweet mint', you inevitably place your order. You get your package in the mail, and it's a nice, brand-new bottle with an intriguing label that has a modern color scheme and text layout, giving you the impression that this stuff is 'professional'.
Combine that with the hand-written "Thank you so much", and you feel that not only is this professional, high quality stuff, but it was made with care just for me. Perhaps they even intentionally put a dab of the recipe on the bottle (unbeknownst to you), so that the aroma fills up your room the moment you open your package, and you think, 'man my recipes don't have such potent aromas'. You take the first few drags, already having approved of the juice before your taste buds get to let you know whether or not they dig it. 'Wow, this is truly great stuff' your brain says, even though you're vaping on nothing more than TFA Juicy Peach, Koolada, and a small % of 50/50 Sweetener/Bavarian Cream with a few drops of EM.
When you mixed this exact recipe a few weeks ago, you made a sketchy little 5mL sample in a chinese-sourced plastic bottle with a hand-written chicken scratch label and couldn't smell much since you didn't intentionally dab the bottle. You vaped it, knowing exactly what was in it and as your mind tried to calculate whether or not you added too much of this or too little of that as you took your sample drags, you never really enjoyed the vape itself and deemed the recipe a failure. Perhaps there were some minor differences. You didn't add the touch of Bavarian Cream or the drop of EM. Maybe you had a high PG ratio and the vendor uses a high VG ratio with distilled water. These may seem like an ocean of difference in your perception, but they're really extremely minute in comparison to what your perception has created.
The same thing happens with the music I make. I'll lay down a real raunchy solo over a track were working on, play it back a few times, and think 'meh'. I'll then listen to a real popular musician's solo later that day, and even though it was just as raunchy and had all the same minor 'mistakes' (finger to fretboard picked up on mic, occasional bass thump when your thumb touches the low E, etc.), my perception thinks "wow what an amazing solo, only a true legend can jam such a solo, flawless". I once tested this theory out.
I recorded a guitar track and solo'd over it, then mastered it accordingly. A few friends were invited to hear it, and they were informed it was a new instrumental track by John Frusciante. They loved it, and asked where I got it. I made a very similar track and invited those same friends a few days later to check it out, asking what they thought of *my* guitar track. They said it was alright, and had a mouthful of recommendations/potential improvements for me.
It's weird, but it's how the brain works. If something is signified as professional, high-grade, finished product, you are quick to accept it and create a sort of mental symbolism that is detached from what the product actually is, whereas if *you* create something, it's easy to dismiss it as 'not good enough' or 'not quite there' since there is no mystery. You know exactly what went into it, and your mind questions and doubts whether or not there is room for improvement, potentially to the point of overlooking the value or legitimacy of your creation.